1. Field of the Invention
The invention relates to ultrafine particles formed of various types of solid materials such as metal, semiconductor and compound and a method for producing the ultrafine particles, a method for producing ultrafine particle bonded bodies, and fullerenes by a novel production method and a method for producing the fullerenes.
2. Description of the Related Art
Metal particles or compound particles such as metal oxide particles, when particles are ultrafine with a diameter of 100 nm or below, they have different properties from normal particles (e.g., 1 .mu.m or larger). In a ultrafine particle, a number of atoms existed on a particle surface increase relative to total number of atoms of the particle. Therefore, since an effect of surface free energy cannot be ignored in consideration of the properties of particles, new properties may be produced.
The ultrafine particles are suitably used to find a new surface phenomenon and to grasp an outline of the new surface phenomenon. For example, a melting point and a sintering temperature of ultrafine particles decrease in comparison with a bulk. In addition, when there are a plurality of ultrafine particles, a tunnel effect may be caused among them, or quantum mechanical effects (such as a quantum well and a mini band) may take place. A high catalytic effect can be obtained depending on types of ultrafine particles. These ultrafine particles can be used to improve the properties of materials and to develop a very fine device, and can also be applied to functional materials such as a catalyst. Physical properties of ultrafine particles and a usage of ultrafine particles are studied.
Conventional ultrafine particles are produced by, for example, physical or chemical methods. The physical methods for producing ultrafine particles include a gas evaporation method, a metal evaporation synthesis method, and a vacuum evaporation method on a fluid oil. In the gas evaporation method, a metal or the like is evaporated in an inert gas and then ultrafine particles are produced to be cooled and condensed by collision of the evaporated metal with the gas. As a material for ultrafine particles, metal atoms evaporated by sputtering may also be used. In the metal evaporation synthesis method, a metal is heated in a vacuum and vaporized, metal atoms are deposited together with an organic solvent on a substrate which is cooled under a freezing point of the organic solvent. In the vacuum evaporation method on a fluid oil, a metal is deposited on an oil.
Chemical methods for producing ultrafine particles are known to utilize a liquid or gas phase. The production methods using a liquid phase include a colloid method, an alkoxide method, a coprecipitation method and the like. In the colloid method, a noble metal salt is reduced in an alcohol coexisted with a high molecular surface active agent under reflux. In the alkoxide method, it is utilized the hydrolysis of metal alkoxide. In the coprecipitation method, a precipitant is added to a metal salt mixed solution to obtain precipitated particles.
The production methods using a gas phase include a thermal decomposition method for organic metal compounds, a metal chloride reducing/oxidizing/nitriding method, a reduction method in hydrogen, and a solvent evaporation method. In the thermal decomposition method of organic metal compounds, a metal carbonyl compound or the like is pyrolized to obtain metal ultrafine particles. In the metal chloride reducing/oxidizing/nitriding method, a metal chloride is reduced/oxidized or nitrided in a reaction gas flow to obtain ultrafine particles. In the reduction method in hydrogen, an oxide or a hydrate is heated in a hydrogen current to reduce. In the solvent evaporation method, a metal salt solution is atomized through a nozzle to dry by hot air.
Conventional research and development of ultrafine particles are mainly related to an aggregate of ultrafine particles. The properties and applications of ultrafine particles and also various ways of operating and controlling the ultrafine particle as a unit substance are less studied because of the methods for producing the above-described ultrafine particles. Namely, an ultrafine particle was hardly obtained as a unit substance by the conventional production methods.
Some studies are being made to apply the ultrafine particles to devices and various functional materials. But, even if the conventional production methods could produce an ultrafine particle as a unit substance, such methods cannot fully control its formed state. Thus, the ultrafine particles are hindered from being applied. For example, it is expected that ultrafine products, various devices and various functional materials can be produced by bonding ultrafine particles mutually under controlled conditions. But, since researches on control of bonding ultrafine particles mutually are insufficient, the above-described applications and developments have not been completed.
To facilitate researches on the physical properties and the applications of the ultrafine particle as a unit substance, it is demanded to achieve the production of an ultrafine particle as a unit substance. It is also required to achieve a technique that can control a position and a state to produce the ultrafine particle. Furthermore, it is desired to complete a technology which can bond the ultrafine particles as a unit substance under controlled conditions and a technology for stabilizing a bonded ultrafine particle which is significant in applying a bonded ultrafine particle.